This invention relates to plastic resin pelletizers and more particularly to apparatus and method for the adjustment of pelletizer knives in relation to the cutting plane or extrusion face of a multiple orifice extrusion die.
Underwater plastic resin pelletizers generally incorporate either manual or motor driven apparatus for adjusting the blades of a knife blade assembly in relation to the cutting surface or face of the extrusion die. It is important to define and maintain this relationship to provide for a prolonged life of the cutting edges of the knife blades and to ensure proper cutting of polymers in an effective manner.
Various control arrangements have been suggested for determining the position of the cutting knives with respect to the die extrusion face, or for determining when the cutting knives have come into physical contact with the die face. One such arrangement, as shown in the Holmes et al, U.S. Pat. No. 4,529,370, senses the onset of electrical conductivity between the rotating knife and the die face. Such an arrangement, which detect the presence or absence of conductivity, requires that the knife mounting shaft and bearings be electrically isolated from the die plate itself. This results in substantial additional costs and complications to the design and construction of the pelletizer.
Arrangements which rely upon the presence of conductivity, to determine the position of the knife, can only indicate that a knife blade is or is not in contact with the die plate itself. They cannot accurately or effectively determine the extent or degree of contact pressure.
A second concept which has been employed for determining blade position resides in the employment of proximity detectors in the die plate as shown in Nagahara et al, U.S. Pat. No. 3,912,434. The conductors or sensors in the die plate are insulated from the nozzles. A pulse counter is used to determine when contact is made, as picked up by the sensors. When all of the knives are contacting, all of the pulses should be present. However, this arrangement does not provide accurate information as to the extent or degree of contacting pressure.
Proximity sensors have the disadvantage of difficulty in adjustment and calibration. Such systems also have the difficulty of having to design the die plate so that the sensors can be embedded therein, and the sensors themselves have to survive the heat of plastic extrusion, the presence of the plastic material, and the presence of water in an underwater pelletizer. An improperly positioned sensor may be cut or damaged by a knife.
A further disadvantage of a proximity sensor system is that the system cannot compensate for die wear. For example, if an operator wants to run his knives at a particular clearance, and sets the sensor system for that clearance, he will be indicating only a knife to sensor proximity. Therefore, when the die plate wears, the effective gap will be increased by the amount of wear, but the sensor will continue to indicate the spacing between the sensor and the knife blades.
A need exists for a pelletizer in which the contact of the knife with the die plate face may be unfailingly detected, and the extent and or pressure of contact may be monitored to permit continuous running contact under low pressure or to permit and provide safely for higher pressure contact for specific purposes, such as for knife blade sharpening.